Of the four common “strokes” used in swimming, being freestyle, backstroke, butterfly and breast stroke, all except breaststroke utilize a dorsal/ventral kicking action where the legs each move substantially in a plane of normal bipedal ambulation. A number of recreational and training aids exist for use with all of the dorsal/ventral-kick strokes including kick boards and hand fins, and swimming foot fins which can be utilised for freestyle, backstroke and butterfly. The style of fin applicable to one of these strokes is easily applied to another, furthermore the same style of fin can be used for scuba diving, snorkelling, boogie boarding, body surfing and fin racing. The common style of fin used in these strokes and sports can be termed a “vertical fin” due to its suitability for use where the stroke uses a dorsal/ventral kicking action. However, due to the distinct differences between the kick mechanics of freestyle, backstroke and butterfly on the one hand and breast stroke on the other it is generally inappropriate to apply a vertical fin to breast stroke.
The use of fins during swimming training has many benefits; it can improve kick strength through increased resistance, increase ankle flexibility thus allowing better angles with which to attack the water during the propulsive phase, and improve body position in the water. Additionally it allows the swimmer to focus on other aspects of their stroke. All of these aid in the development of improved kick strength and stroke technique. Furthermore, because fins allow the maintenance of higher speeds for longer periods of time they allow the swimmer and to increase overall muscle strength and endurance, cardiovascular fitness and flexibility.
Generally the swimming style utilised by a swimmer at race pace is different to that at lower speeds. Athletes aim for “motor memory”, by which the desired style/technique is learnt at a muscular level. This means that as the swimmer tires, they can maintain these technical principles longer. Small fins allow the swimmer to reach race pace speeds without the same amount of exertion and also without altering their normal kicking style, thus allowing the swimmer to concentrate on arm stroke technique at high speeds. Similarly if the swimmer has an upper body injury, by using fins he/she will benefit from still being able to train at race speed with less strain being placed on the upper body in order to maintain that pace.
Russian Inventor's Certificate SU 1227213 discloses a breaststroke swimming aid having a body portion including a foot pocket and an integrally formed lateral resilient web extending laterally of the foot. The essence of this apparatus is to increase the effective area of the sole of the foot in order that an increased muscular effort may be converted to propulsive force on the power stroke of the kick. However, the apparatus similarly, imposes an increased area opposing the return stroke, which operates counter to the low resistance return stroke of the unaided breaststroker, forcing an exaggerated action to feather the web.
German Patent Specification DE 460511 describes a breaststroke swimming aid having a foot supporting body having hinged, rigid flaps extending laterally and inward of the sole of the user. The flaps are adapted to fold under water resistance along the hinge lines to reduce drag on the return stroke and to deploy to blocked, respectively lateral and inward extended positions on the power stroke to increase the effective area of the sole of the foot in order that an increased muscular effort may be converted to propulsive force. This construction has the disadvantages of interference between the respective inward flaps on the left and right feet of the user at the end of the power stroke. The disposition of a flap to the inner side of the foot also tends to cause eversion of the foot of the user in the power stroke. The hinged and blocked arrangement is not progressive and shock-loads the user's ankles early in the power stroke.